How to debug AN's red boundary box?
Red border means a fatal error occured. This is probably caused by a bug in Animation Nodes, however, it may also be an error in your node tree when you disable certain error checking options. In case of a fatal error:


A new panel called the Problems Panel appear. It include some suggestions and an operator to open a new issue on github to report the error as a bug. Choosing Monitor Execution as the Execution Code method in the Developer Panel should present more information in the problems panel like the node that causes the problem.
The error will be printed to the terminal, so looking at the terminal should give you all the information you need, like the node that caused the problem and what exactly is the problem.


It should be noted, however, that some nodes don't have an execution function, and they contribute code to the execution unit directly, thus the terminal output will not be as useful. In this case, the terminal output should be something like:

  File "/home/omar/projects/blender2.80/2.80/scripts/addons/animation_nodes/execution/main_execution_unit.py", line 38, in executeUnit
    exec(self.executeCodeObject, self.executionData, self.executionData)
  File "execution: 'NodeTree'", line 3, in &lt;module&gt;
  File "&lt;string&gt;", line 11, in main


Which tells us that the error occurred in line 3 of the execution code of NodeTree. The Developer Panel have an operator that prints/write the execution code of the selected tree to the terminal/text block, the output should be something like this:

 ...
  9.  nodes = bpy.data.node_groups['NodeTree'].nodes
 10.  _bqracm2qds5y5ta = nodes['Expression']
 11.  __socket_False_0_bqr = None


  1.  
  2.  # Node: 'NodeTree' - 'Expression'
  3.  _result_bqr1 = _bqracm2qds5y5ta.expressionFunction()


Line 3 of the execution code lies in the scope of the node called Expression, so we know that this node is the source of the problem.